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From messy wave to hurricane overnight. Why computer models missed on Oscar forecast

Updated: 22-10-2024, 02.36 PM

On Friday night, a messy tropical wave to the east of Puerto Rico had just a 10% chance of strengthening over the weekend. By lunchtime on Saturday, it was a Category 1 hurricane bearing down on the Bahamas.

What happened?

Experts said the tiny storm ducked the attention of most major storm models, but humans watching the data pour in — and pilots and researchers gathering it themselves — were able to sound the alarm before Hurricane Oscar made landfall.

Philippe Papin was the forecaster on duty for the National Hurricane Center Saturday morning. He first spotted a problem while closely inspecting the passive microwave imagery, a satellite that offers scientists a view of what’s happening underneath the clouds. He spotted a low-level swirl, a tell-tale sign of a tropical storm.

“It became pretty clear that a small circulation was developing,” he told the Miami Herald. “We had to shift gear in a short period of time.”

By 11 a.m., the hurricane center issued its first forecast for Tropical Storm Oscar, complete with a cone pointed right at the Bahamas and Cuba. The Bahamas issued a tropical storm warning.

At the same time, a quickly pulled-together crew of Hurricane Hunters took to the skies from the island of St. Croix.

Within an hour and a half, they found a very different system than they saw a few days earlier, north of Puerto Rico. And it was easy to miss. The plane didn’t register tropical-storm-force winds until they were 10 nautical miles from the center, Papin said.

By 2 p.m., Tropical Storm Oscar was Hurricane Oscar, one of the smaller hurricanes on record in the Caribbean. That gave those islands less than a day to prepare for an imminent hurricane.

“The typical time for issuing a watch is 48 hours of lead time. This was more like 12 to 24 hours. Obviously that is sub-optimal,” Papin said.

Hurricane Oscar made landfall on Great Inagua Island in the Bahamas Sunday morning and on the eastern coast of Cuba on Sunday evening.

What the models missed

The system that would eventually become Oscar rolled off the coast of Africa more than a week earlier. Initially, computer models picked up the system and gave it a decent chance of forming a tropical depression — or something stronger.

But a surge of very dry air sucked the life out of it, or so the computer models thought. Hurricane hunter reconnaissance later in the week also found nothing but a tropical wave. On Friday, no major hurricane models showed the chance of a tropical storm forming anywhere in the Caribbean or Atlantic in the next seven days.

By Saturday, it was a different story.

“I think the models just had a hard time resolving the circulation before they got the recon in there,” said Phil Klotzbach, senior research scientist at Colorado State University and author of its respected pre-seasonal forecast. “It’s not like the models didn’t have signals, they had them and then it killed them off.”

The data collected on the reconnaissance flight was fed into the array of computer models shortly after, and by mid-afternoon, Papin said, they were catching up with Oscar. They showed a very small storm, with hurricane-force winds extending only 5 nautical miles from the center.

“Size is definitely an important part of the equation of why the models weren’t handling this storm so well,” Papin said.

Oscar was definitely a small storm, but doesn’t quite match up to the records other storms have set since the hurricane center started tracking the average radius of tropical storm-force winds in 2004 . Klotzbach said the smallest storms were Humberto in 2007, with a radius of 26 nautical miles and Jeanne in 2004 with a radius of 28 nautical miles.

Oscar had a radius of 34 nautical miles when it was first designated a hurricane on Saturday.

“Even though it’s low, they always had a 10% chance. You just never know. It’s a tough forecast,” Klotzbach said. “These small storms are tricky.”

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